McIntosh MR-80 Brochure

The Mclntosh MR 80

THE ULTIMATE DIGITAL FM TUNER

The MR 80 tuner has the most flexible control

System ever designed in a stereo FM tuner

Engineering direction dictated a tuner design
governed by insistence on great flexibility and ease
of use. This had to be done while designing an RF
section with great sensitivity and the world's best
selectivity in keeping with the needs of low distor­tion. These values were achieved and they retain the
Mclntosh reputation of outstanding performance,
long life and reliability.

Tuning in a station on the MR 80 is achieved elec­tronically. This assures long term trouble free opera­tion. You can tune a station on the MR 80 four dif­ferent ways.

1. Manual tuning by rotating the main tuning knob.

2. Auto Scan automatically searches for the next

available station up or down in frequency.

3. Presets - allows you to select your four most

listened to stations that can be tuned at the touch

of your finger tip.

4. Remote Scanning will allow the MR 80 to be tuned

to a station from a remote location.

Two separate antenna systems can be connected

to the MR 80.

1) An outdoor or indoor FM antenna, or 2) a cable in­put from your local cable company. The antenna
selection is controlled by an electronic switching

device containing more than 75 dB of isolation and
no signal loss as might normally be found in exter­nal mechanical switch.

The MR 80 uses electronic varactor tuning instead
of the more conventional mechanically gauged
variable tuning capacitors. Variable tuning
capacitors can with age collect dust and dirt, reduc­ing their performance. Mclntosh uses double varac­tor diodes to provide the necessary tracking be­tween the different tuned RF stages. When a weak
distant station is adjacent to a strong local station
the Preselector circuit will switch in an additional
tuned circuit providing an extra degree of selectivity
reducing the interference from the adjacent strong
station.

After the RF amplifier two paralled tuned circuits
are used to provide the proper load impedance for
the bipolar transistor. These two tuned circuits
greatly improve the image rejection and overload

performance of the tuner, as well as increasing the
RF selectivity.

An innovative new lock circuit was developed for
use in the MR 80. This new circuit allows correct tun­ing without the use of a center tune meter. The MR

80 will be correctly tuned even if the station of the

cable company is not on its correct frequency. This
is done by the use of two operational amplifiers. The
lock circuit will track a station even if it drifts 1 MHZ.

The mixer is a balanced matched dual J-FET and
bipolar transistor circuit. A low loss toroidal phase
splitting transformer is used as an impedance mat­ching network in the gate circuit of the mixer
eliminating even order spurious responses.

The MR 80 has the narrowest IF bandwidth ever
used in a stereo tuner. It is the correct width to let
just one FM station through. The excellent selectivi­ty of the MR 80 (210 kHz wide at 60 dB down) permits
tuning to stations that are impossible to receive on
ordinary tuners.

After the mixer the signal is electronically switch­ed to go either directly to the IF amplifier or to go
through a quartz crystal filter. The SUPER NARROW
selectivity position adds a 4 pole - 4 zero crystal filter
to the other 5 IF filters. SUPER NARROW permits
receiving stations never heard before with most
other FM tuners. The 5 stages of IF amplification
provide the necessary gain to reduce noise in the
signal and interference from other stations. The 5 IF
stages use piezoelectric fixed frequency filters in
place of normal tuned circuits so the IF stages will
always stay in alignment, even with age.

The signal strength indicator column is summed
off of all 5 IF stages instead of just 1 so it is looking
at the signal strength throughout the entire IF
system.

Following the selectivity section of the IF
amplifier is the LIMITER. A total gain of 80 dB is
used in this circuit. The use of very high gain in the
limiter circuit produces hard limiting with very good
impulse noise rejection. Limiter bandwidth is
greater than 50 MHz, producing excellent detector
capture characteristics.

A broadband Foster-Seeley discriminator is used
as the detector. This detector coupled with the
broadband limiter produces unmeasurable noise
and distortion.

The heart of the multiplex section is a new third
generation phase locked loop (PLL) stereo decoder

integrated circuit (IC). This PLL IC incorporates two
special systems, an automatic variable separation
control circuit to reduce background noise when
receiving weak stereo stations, and tri-level digital
waveform generation which eliminates interference
from SCA signals and from the sidebands of adja­cent channel FM signals.

The variable separation control is operated from
the IF amplifier's signal strength detector. A smooth
transition is provided from mono to stereo or from
stereo to mono at weak signal levels to provide the
optimum signal to noise ratio and best stereo
separation for the prevailing signal conditions. The
circuit operates only during stereo reception. It swit­ches automatically to monophonic if the 19 kHz pilot
tone is absent.

In the PLL the internal oscillator operates at
228kHz, locked onto the 19kHz pilot tone. The
228kHz, feeds a 3 stage Johnson counter via a binary
divider to generate a series of square waves.
Suitably connected NAND gates and exclusive OR
gates produce the tri-level drive waveform for the
various demodulators in the circuit. The usual
square waveforms have been replaced in the PLL
and decoder sections by tri-level waveforms. These

tri-level forms contain no harmonics which are
multiples of 2 or 3. This eliminates frequency
translation and detection of interference from the
side-bands of adjacent stations since the third har­monic of the sub-carrier (114kHz) is excluded. It also
eliminates interference from SCA broadcasts since
the third harmonic of the pilot tone (57kHz) is exclud­ed. Unwanted spurious audible components and
phase jitter in the PLL are Inherently eliminated by
this technique.

Additional advantages of the phase locked loop

stereo demodulation are the elimination of induc­tors to minimize drift, integral lamp driving capabili­ty to indicate the presence of the 19kHz pilot carrier,

excellent channel separation over the entire audio

frequency range, extremely low distortion, low out-

put impedance, and transient-free mono/stereo

switching.

Following the MPX decoder is the three position
de-emphasis switch. The three different positions
allow the MR 80 to be used in North America with
standard 75ms de-emphasis and in Western Europe
and the Far East with 50ms de-emphasis. The 25ms
position is for use with an external noise reduction
adapter.

An electronic blend filter circuit, implemented
with two J-FETs of a quad J-FET package, is used to

reduce out of phase noise when in the stereo mode
and tuned to a weak station. This filter is actually a
twin-T bandpass that blends the high and low fre­quencies, leaving separation unaffected at mid­frequencies. This results in a greatly improved
stereo image when the filter is required.

Special design attention has been given to the

power supply section. Nine separate power circuits
are used. Six of these are regulated to prevent loss
of performance during a brown out. The - 15, - 5.2,
5, 15 and 30v use integrated circuit 3 terminal

regulators, while the 3ma current source is made
with discrete transistors because of the high voltage
on the input terminal. The remaining voltages are

used for the headphone amplifier and the touch con­trol reference signal driver.

Mclntosh, recognizing the need for improved

reliability, has designed a new circuit to drive in­candecent lamps. This new circuit prevents the
filaments from failing due to notching when
operated on direct current. This failure mode can

reduce lamp life from one half to one tenth of the

data sheet value. In the MR 80 the three lamps that

are used for STEREO, LOCK and FILTER indicators

are operated on AC at lower than rated voltage to ex-

tend the useful life to well in excess of 15 years.

Only Mclntosh brings you this feature.

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